1. I don't think there are many routers out there that allow you to boost the power to the wireless connection, and though I have no experience with them, I doubt the Airport Extreme will contain such an option. I will let somebody else answer this more definitively.

2. Is it that you simply want it to backup once a month or that you only want monthly backups available for the future? If it's the latter, then you should know that how Time Machine works is that it'll backup your system hourly, then consolidate them into daily backups, then weekly, and when you've have the backup running for a while, monthly. So ultimately it'll end up being monthly anyway. However, if you're looking to do the former, then I believe there is a Terminal command, though I'm not sure exactly what it is.